WASHINGTON—The
Republican-controlled Senate on Tuesday took its first step toward
dismantling the 2010 Affordable Care Act, using its initial day in
office to introduce a measure that sets an aggressive timeline for
developing plans to repeal much of President Barack Obama’s
signature health law.

Republicans have
long said that they would move toward repealing the health law on Day
One if they returned to power, arguing that the law is collapsing as
some premiums surge and many insurers on the exchanges created by the
law stop writing policies. The Senate action came shortly after
lawmakers were sworn in for the new session of Congress.

The action came on a
day that showed both discord and unity among Republicans at a moment
when they will have control of both Congress and the White House for
the first time in a decade.

Creating unexpected
friction, House Republicans triggered a battle with House Speaker
Paul Ryan and President-elect Donald Trump by advancing and then
abandoning a plan to reduce the independence of a House office that
investigates ethics violations. House GOP lawmakers said the office
had acted unfairly, but Mr. Ryan opposed the move, and Mr. Trump
urged the party to turn toward other priorities.

THE TRUMP TRANSITION

Rep. Paul Ryan (R.,
Wis.) won re-election as House speaker Tuesday in a 239-189 vote that
showed House Republicans to be unified as their party takes the reins
of power in Washington. Photo: AP

Shortly after the
proposed ethics plan collapsed, Mr. Ryan won re-election as House
speaker with the support of all but one Republican in the chamber, a
show of unity after a fractious election season. Nine Republicans had
voted against Mr. Ryan in protest in October 2015, when he succeeded
former Rep. John Boehner as speaker, and a number of conservatives
had threatened last year to pull their support from him for declining
to campaign during the general election with Mr. Trump.

The first fireworks
over the repeal of the 2010 health law will begin Wednesday, when
Senate Majority Leader Mitch McConnell (R., Ky.) is set to open
debate on a measure known as a budget resolution. If approved as
expected, it would set in motion a controversial legislative process
allowing the GOP to repeal much of the health law with a simple
majority vote. Republicans are using the process because they hold 52
seats in the new Senate, short of the 60 needed to approve most
legislation.

The Senate debate
will take place against a backdrop of high-profile lobbying.
President Barack Obama is set to huddle with House and Senate
Democrats on Wednesday to discuss strategies to protect the law. Vice
President-elect Mike Pence plans to meet separately with House and
then Senate Republicans to discuss the GOP’s own plans.

A repeal effort
could create upheaval in the insurance markets and the loss of
coverage for millions of Americans. It could also could spark a
backlash in the 2018 elections.

Republicans have
seized on their control of the major levers of power in Washington to
deliver on a key campaign pledge to repeal the law. But while
starting the repeal machinery, they don’t yet know their endgame.

It remains unclear
whether lawmakers will follow through on eliminating the ACA without
having a replacement for a law that delivered health coverage to
roughly 19 million previously uninsured people, mostly through an
expansion of the Medicaid program for low-income Americans, and that
sharply lowered the U.S. uninsured rate. What might replace the law
remains undetermined.

The Senate’s
budget resolution directs four relevant committees, two in the Senate
and two in the House, to write legislation by Jan. 27 that reconciles
spending and tax policy with the budget blueprint for the coming
fiscal year. Embedded in the committees’ legislation will be
provisions that repeal much of the health law.

The result will be a
single piece of legislation known as a reconciliation bill—a
privileged piece of legislation that can be approved in the Senate
with a simple majority.

The House has
already voted 89 times to repeal the law, but its efforts have been
stymied by the Senate.

In 2015, the Senate
used the same procedure to repeal large parts of the Affordable Care
Act, and by 2016 the House had passed the measure. Mr. Obama vetoed
it. Congress sealed passage of the 2010 health law by passing a
related measure that also relied on the procedural tactic.

Mr. Ryan, in a
statement, said the new Senate measure “sets the stage for repeal,
followed by a stable transition to a better health care system.’’

Democrats said that
it was up to the GOP to lay out what would replace the health law.
Even the most vulnerable Democrats—centrist lawmakers up for
re-election in conservative states in 2018—said they wouldn’t go
along with the GOP plan to repeal the health law before some
replacement was ready.

“I’m not opposed
to repealing as long as I had something that would replace it,”
said Sen. Joe Manchin (D., W.Va.). “But right now, there’s
nothing on the table.”

Rep. Kevin Cramer
(R., N.D.), a member of the Energy and Commerce panel, said the most
pressing question would be how long to make the transition to a new
health-care system. “The biggest issue for all of us on Obamacare
is going to be the speed with which we replace it,” said Mr.
Cramer.

The budget
resolution and reconciliation process can be used to repeal only
portions of the Affordable Care Act, among them provisions that
require individuals to obtain coverage, that provide tax credits for
people to buy coverage, and that direct employers to offer insurance
or pay penalties.

It isn’t expected
to include measures that would replace the health law with a new
insurance program. Some Republicans have said they want to repeal the
law in order to create political pressure on lawmakers to write a
replacement plan, but the details of such a plan have yet to be
determined.

“Obamacare
over-promised and under-delivered,” Sen. Chuck Grassley (R., Iowa)
said in a statement.

“At every step of
this process, I plan to keep in mind how Congress and the incoming
president can best deliver for the millions of Americans counting on
us to act to repeal Obamacare while protecting access to medical care
for all Americans,” he said.

House Minority
Leader Nancy Pelosi (D., Calif.) countered: “The purpose of the
Affordable Care Act was to lower costs, to improve benefits and to
increase access.” She added that “the ACA has made
transformational progress in doing all three.”

—Stephanie Armour
contributed to this article.

Write to Siobhan
Hughes at siobhan.hughes@wsj.com and Kristina Peterson at
kristina.peterson@wsj.com